The subject matter disclosed herein generally relates to chillers, and more particularly, to a method and apparatus to control harmonics of variable frequency drives of chillers.
An active harmonic filter (AHF) is an analog or digital device that is installed on a coupling point to a line in parallel with a rectifier front end type variable frequency drive and is configured to measure power quality on the grid or the rectifier side and to inject current from the coupling point to the electrical grid accordingly. That is, the AHF injects the current into the line in order to compensate for any unwanted deviations from the standard current (i.e., 50 or 60 Hz supply) as found from the measurements. Such deviations can be partially mitigated or mitigated in full.
In some cases, such as those where the load includes a rectifier front end type variable frequency drive in high tier chiller applications, the deviations may be present in the form of current harmonics. Here, when an active harmonic filter (AHF) is used to reduce current harmonics on the AC grid side connected in parallel with rectifier front end type variable frequency drives for high-tier chiller applications, it normally achieves best total harmonic cancellation (THD) performance at full load conditions. The THD level will increase at partial load conditions due to higher current distortion ratio at partial load conditions